This invention relates to a method of coating a substrate with a resist, such as a photoresist, an electron beam resist, or the like to manufacture an article which can be processed into a reticle, a mask, or the like.
A reticle and a photo-mask are indispensable with large scale integration or very large scale integration. Such a reticle or a mask is manufactured by delineating a pattern on a substrate, such as a mask blank, by the use of a lithography technique. Before delineating such a pattern, the mask blank should be coated with a resist film. It is troublesome for each customer to coat the mask blank with the resist film. Accordingly, the customer's demands have been recently directed to an article having the resist film coated on the substrate.
The substrate may be either circular or square in shape. At any rate, the resist film should uniformly be coated irrespective of the shape of the substrate.
A conventional method is implemented by the use of a spin coater, as will later be described with reference to one figure of the accompanying drawing. More particularly, the resist is dripped on the substrate supported on a rotatable chuck of the spin coater. The substrate is thereafter rotated or spun together with the chuck. As a result, the resist is spread or expanded on the substrate to form the resist film on the substrate. When the resist spreads on the substrate, an interference color visibly appears and moves from a central portion of the substrate to a peripheral portion thereof. Such a movement of the interference color stops when the resist is dried into the resist film. In addition, the thickness of the resist film is dependent on the speed of rotation of the substrate.
Under the circumstances, the conventional method forms the resist film to a desired thickness by controlling the speed of rotation and by observing the movement of the interference color. The speed of rotation is invariable in the conventional method until the movement of the interference color stops on the substrate.
However, the resist film inevitably becomes nonuniform in thickness with the conventional method. Especially, such nonuniformity becomes serious when the substrate is square in shape.
Alternatively, another coating method is proposed by K. Shibata in Japanese Unexamined Patent Publication No. Syo 58-207,631, namely, 207,631/1983. The proposed method can form a uniform resist film on a circular substrate by the use of the spin coater. Specifically, the speed of rotation of the substrate is varied at a first, a second, and a third step which serve for spreading the resist on the substrate, for expelling a superfluous amount of the resist from the substrate, and for stabilizing the resist into the resist film, respectively. The speed of rotation is faster during the second step than the third step.
However, the resist film also becomes nonuniform in thickness with the proposed method when the substrate is square in shape.